Display devices of a cathode ray tube (CRT) have economically been mass-produced owing to the progress and development of television technologies achieved up to this date, as is well known in the art. In recent years, however, a thin and lightweight flat panel display has drawn an increasing attention as a display device of a next generation that will replace the display devices of the CRT.
One of the typical examples of such flat panel displays is a liquid crystal display (LCD), and the LCD has already been used as a compact display device of notebook type personal computers, cellular telephone sets, personal digital assistants (PDA) and other mobile electronic information devices. On the other hand, a plasma display panel (PDP) is a typical example of thin and large-scale flat panel displays. This plasma display panel has practically been used as wall-hung television receivers for business or home use.
A typical PDP includes a large number of small discharge display cells. In general, each discharge display cell is encompassed and defined with a pair of glass substrates opposing each other in a spaced-apart relation and a rib of a microstructure having a predetermined shape and interposed between these glass substrates. Electrodes are arranged in a spaced-apart relation on the inner surfaces of the glass substrate and are patterned. A rare gas is filled into each discharge display cell so that desired self-light emission can be effected by plasma discharge between the electrodes. Therefore, the PDP is substantially free from field-of-view angle dependence.
The rib described above is generally formed of a ceramic microstructure, is arranged in advance on the back of the glass substrate and constitutes a part of the PDP back plate. In this instance, the PDP back plate mostly has the ribs having the shapes broadly classified into the following two types. One of them is a shape called “straight pattern”, and is described in International Publication Gazette No. 00/39829, for example. This straight pattern is simple and can relatively easily manufacture PDP having a large scale.
As described in International Publication Gazette No. 00/39829, a flexible resin mold can be used to mold the rib having the straight pattern. The resin mold is manufactured in the following way. First, a photosensitive resin is filled into a metal master mold having a pattern and a shape corresponding to those of the resin mold, that is, into the metal master mold having the same pattern and the same shape as those of the rib to be manufactured. Next, this photosensitive resin is covered with a plastic film and is cured to integrate the photosensitive resin after curing with the film. The film is then released with the photosensitive resin from the metal master mold.
Here, the photosensitive resin has a high viscosity of 500 to 5,000 cps. This is for suppressing shrinkage of the photosensitive resin upon curing. When the photosensitive resin having such a viscosity is used, the photosensitive resin can be filled without entrapping air bubbles between the metal master mold and the film.
Another rib has a shape called “lattice pattern”. The lattice pattern can suppress much more the drop of vertical resolution of PDP than the straight pattern. For, ultraviolet rays from the discharge display cell are more difficult to leak outside. When compared with the straight pattern, the lattice pattern can keep light emission efficiency from the display discharge cell at a higher level. Another reason is that a phosphor necessary for color display of the PDP can be applied with a relatively greater area to the discharge display cell.
A mold can be used to manufacture the rib having the lattice pattern, too. For example, Japanese Unexamined Patent Publication (Kokai) No. 11-96903 describes a method that pushes a rib material into a metal master mold by use of a vacuum press molding machine, cools the rib material and then withdraws it from the metal master mold. However, since the size of the vacuum press molding machine is limited, it can manufacture a back plate for PDP having a size of only a few cm, and is not suitable for manufacturing PDP to serve as a large display. Japanese Unexamined Patent Publication (Kokai) No. 9-283017 discloses the use of a cylindrical metal master mold having an opposite pattern to that of a lattice. This metal master mold moves and turns on a substrate through a barrier member and pushes the barrier member to the substrate. In this way, the metal master mold can manufacture the rib having the lattice pattern. Generally, however, the barrier member is by far softer than the metal master mold. In consequence, when the rib is released with the substrate from the metal master mold, the rib is likely to be broken. Breakage of the rib is particularly remarkable in a substrate having the ribs in a direction vertical to the rotating/moving direction of the metal master mold.
If the flexible mold described above can be applied to molding of the rib, breakage of the rib may be avoided. According to the existing molding technology, however, it is difficult to manufacture such a mold. For, as typically shown in FIG. 10(A), when a photosensitive resin 2 having a high viscosity of 500 to 5,000 cps is filled between the mold 5 and the plastic film 1, it is difficult to fill the photosensitive resin 2 without entrapping bubbles 12. When the photosensitive resin 2 is photo-cured while containing the bubbles 12, the bubbles 12 remain as such inside and on the outer surface of the photosensitive resin 2 after curing as shown in FIG. 10(B). Such bubbles 12 result in rib defects when the ribs are manufactured by using the mold. Therefore, it is preferred not to entrap the bubbles as much as possible into the mold.
A photosensitive resin having a high viscosity such as described above can be filled into the metal master mold without entrapping the bubbles if vacuum equipment such as a vacuum press molding machine is employed. As described in Japanese Unexamined Patent Publication (Kokai) No. 11-96903, however, the size of vacuum equipment is generally limited. Therefore, a mold having a side of only a few cm can be manufactured, and this means is not suitable for manufacturing PDP as a large display.
A photosensitive resin having a low viscosity of not higher than 500 cps may be filled into a metal master mold without entrapping the bubbles even when vacuum equipment is not used. On the other hand, it may be difficult to manufacture a mold into a desired shape because of curing shrinkage that cannot be neglected. When the photosensitive resin 3 having a low viscosity is filled between the metal master mold 5 and the plastic film 1 as typically shown in FIG. 11(A), it is easy to fill the resin 3 without entrapping the bubbles. However, when this photosensitive resin 3 is photo-cured, voids 13 develop between the photosensitive resin 2 after curing and the metal master mold 5 due to a large curing shrinkage ratio of the resin, inviting thereby deformation of the pattern. This pattern deformation is remarkable particularly when a metal master mold generally having a high aspect ratio of projections corresponding to the ribs is used to manufacture a mold such as a rib mold for PDP. Therefore, the mold manufactured by using a photosensitive resin having a low viscosity is not expected to form relatively easily high-quality ribs of the lattice pattern over a broad range on the PDP back plate.